Induced stress and natural fracture mapping play important roles in well stimulation of shale gas, which determine hydraulic fracture geometry and complexity. At present, there are two methods to calculate the induced stress including analytical model and discontinuous displacement method. The analytical model is based on planar fracture assumption without considering of natural fracture impact. The DDM method is widely used to calculate induced stress by non-planar fractures.
In this paper, we designed a triaxial multi-fracture induced stress test apparatus to measure induced stress and revealed hydraulic fracture propagation. The artificial rock with different types of 3D printed natural cracks was employed to simulate impact of bedding, vertical fracture, and complex natural fracture. A series of laboratory tests to investigate the laws of multi-fracture induced stress under a triaxial stress condition. The effects of multiple factors such as fracture geometry parameters, fracture assembly types and pump injection rate on induced stress were also evaluated. Lastly, the fractal dimension method was used to quantitatively evaluate the morphology of fracture surface and fracture network complexity.
The experimental results showed that different combinations of pre-existing natural fractures can effectively reduce the fracture pressure by 25~45%. Compared with the failure test without presetting natural fractures, the induced stress at the wall position of the rock mass decreased to highly 30%. Based on the experimental test data, the different fracture induced stress ratio coefficients of shale reservoir induced stress prediction model before and after fracturing were obtained, and the coincidence rate between the calculated results and the experimental test results was verified to be 91.5%.
This study provides a feasible method for the testing and analysis of fracture induced stress, which is more suitable for the actual characteristics of shale reservoirs and thus provides the experimental basis for the accurate calculation of the induced stress between fractures in the phenomenon of well interference in shale reservoirs.